linux-kernel - Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area

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Date:   Fri, 8 Sep 2023 01:51:06 +0000
From:   HAGIO KAZUHITO(萩尾　一仁) 
        <k-hagio-ab@....com>
To:     Baoquan He <bhe@...hat.com>, Uladzislau Rezki <urezki@...il.com>
CC:     "lijiang@...hat.com" <lijiang@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Dave Chinner <david@...morbit.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>
Subject: Re: [PATCH v2 4/9] mm: vmalloc: Remove global vmap_area_root rb-tree

On 2023/09/07 18:58, Baoquan He wrote:
> On 09/07/23 at 11:39am, Uladzislau Rezki wrote:
>> On Thu, Sep 07, 2023 at 10:17:39AM +0800, Baoquan He wrote:
>>> Add Kazu and Lianbo to CC, and kexec mailing list
>>>
>>> On 08/29/23 at 10:11am, Uladzislau Rezki (Sony) wrote:
>>>> Store allocated objects in a separate nodes. A va->va_start
>>>> address is converted into a correct node where it should
>>>> be placed and resided. An addr_to_node() function is used
>>>> to do a proper address conversion to determine a node that
>>>> contains a VA.
>>>>
>>>> Such approach balances VAs across nodes as a result an access
>>>> becomes scalable. Number of nodes in a system depends on number
>>>> of CPUs divided by two. The density factor in this case is 1/2.
>>>>
>>>> Please note:
>>>>
>>>> 1. As of now allocated VAs are bound to a node-0. It means the
>>>>     patch does not give any difference comparing with a current
>>>>     behavior;
>>>>
>>>> 2. The global vmap_area_lock, vmap_area_root are removed as there
>>>>     is no need in it anymore. The vmap_area_list is still kept and
>>>>     is _empty_. It is exported for a kexec only;
>>>
>>> I haven't taken a test, while accessing all nodes' busy tree to get
>>> va of the lowest address could severely impact kcore reading efficiency
>>> on system with many vmap nodes. People doing live debugging via
>>> /proc/kcore will get a little surprise.
>>>
>>>
>>> Empty vmap_area_list will break makedumpfile utility, Crash utility
>>> could be impactd too. I checked makedumpfile code, it relys on
>>> vmap_area_list to deduce the vmalloc_start value.
>>>
>> It is left part and i hope i fix it in v3. The problem here is
>> we can not give an opportunity to access to vmap internals from
>> outside. This is just not correct, i.e. you are not allowed to
>> access the list directly.
> 
> Right. Thanks for the fix in v3, that is a relief of makedumpfile and
> crash.
> 
> Hi Kazu,
> 
> Meanwhile, I am thinking if we should evaluate the necessity of
> vmap_area_list in makedumpfile and Crash. In makedumpfile, we just use
> vmap_area_list to deduce VMALLOC_START. Wondering if we can export
> VMALLOC_START directly. Surely, the lowest va->va_start in vmap_area_list
> is a tighter low boundary of vmalloc area and can reduce unnecessary
> scanning below the lowest va. Not sure if this is the reason people
> decided to export vmap_area_list.

The kernel commit acd99dbf5402 introduced the original vmlist entry to 
vmcoreinfo, but there is no information about why it did not export 
VMALLOC_START directly.

If VMALLOC_START is exported directly to vmcoreinfo, I think it would be 
enough for makedumpfile.

Thanks,
Kazu